Styrene-based resins represented by rubber-reinforcement styrene-based resins such as an ABS resin have been used in a wide range of fields including home electric appliances, office automation machines, and general merchandise, because the styrene-based resins have excellent mechanical properties, molding processability, and appearance trait. Particularly in a field of automobiles and motorcycles, the styrene-based resins are used in not only interior parts but also exterior parts because of advantages of light-weight, and in this case they are often subjected to coating.
Generally, in a styrene-based resin, it is known that coating resistance is improved by increasing a content of a vinyl cyanide monomer of a styrene-based copolymer in the styrene-based resin, and some methods of improving coatability by increasing an acrylonitrile content of an acrylonitrile-containing copolymer in an ABS resin, that is, by broadening a composition distribution of acrylonitrile, have been investigated. For example, in Patent Documents 1 to 3, there is proposed a method for specifying the acrylonitrile content in a rubber-containing graft copolymer and the acrylonitrile content in a styrene-base copolymer not containing rubber components. However, in this method, coatability and color stability at the time of melting are insufficient and it is necessary to polymerize, mix and extrude several kinds of styrene-base copolymer components having different acrylonitrile contents, and there is a problem that the productivity of ABS is significantly deteriorated.
In Patent Document 4, a copolymer produced by using suspension polymerization of a batch type is employed in order to broaden the composition distribution of acrylonitrile in a styrene-based copolymer, but since this method uses water, it requires separation, cleaning and drying of a polymer and is high in production cost. Moreover, in the suspension polymerization, since acrylonitrile remains excessively late in the polymerization because of limitations of reactivity ratio, polyacrylonitrile is produced and color tone is degraded, and a desired composition distribution cannot be achieved. Furthermore, the method includes an environmental problem that waste water of the steps contains acrylonitrile since unreacted acrylonitrile is dissolved in water.
In Patent Document 5, there is disclosed a method in which by forming an AS resin by the continuous polymerization using an apparatus which employs a hydrostatic mixer, polymerization stability at the high degree of polymerization, which is a problem for solution polymerization of an AS resin, can be improved, and an AS resin having excellent molding processability can be produced, but there is a problem that the polymerization cannot be stabilized due to high viscosity when the continuous polymerization is performed in a region of high vinyl cyanide monomer, and there is no report on the producing technology of broadening the composition distribution of acrylonitrile by use of continuous-type reactors.